The comparatively short lived playing characteristics of tennis balls is a problem that has long been recognized. The problem has two major facets. There is an economic consideration of having to replace tennis balls which are clearly not playworthy because they are worn, and there is the problem of guessing as to when a ball has become so worn that its playing characteristics have been altered. Various inventors have patented different techniques for solving this problem.
U.S. Pat. No. 2,023,672 to Ellis discloses a tennis ball fabric covering having warp and weft threads made of wool yarn. A covering of loosely woven thick yarns are "floated" on top of a covering of closely woven thin yarns. In the fulling operation of the tennis ball, the yarn of the closely woven thin yarns supplies all of the fiber to compact and felt the covering. The thick yarns are arranged in a number of sguares on top of the layer of closely woven thin yarn. The pits bounded by the sguared ribs of the thick yarn are filled with felted fiber from the thin yarn. As the felted fiber from the thin yarn is removed during play, the thick yarn works up its fiber to form a fuzzy nap on the ribs. The fiber filled pits of thin yarn gradually are worn away from the surface of the cover and the thick yarns increase in fuzziness and maintain the desired nap over the covering. The thick yarns thereby constantly renew the desired fuzzy nap of the covering during the removal of the thin yarns over extended periods of play.
U.S. Pat. No. 1,376,778 to Orr discloses locating a series of circular depressions in the fabric cover of a tennis ball to keep the cover securely united with the rubber center. A sealing means used for uniting the fabric cover with the rubber center of the tennis ball is thereby protected from frictional contact with a racguet and with the ground by the depressions of the fabric cover. The circular depressions are distributed over the entire surface of the fabric cover.
U.S. Pat. No. 4,284,276 to Worst discloses a golf ball with a series of dimples and elongated grooves formed on the outer surface of the ball. The dimples and grooves interlock with complementary grooves formed on the face of a golf club. The grip on the ball by the club and "hang time" of the ball are thereby increased.
Today there is no known engineered design for a tennis ball which assists in the flight path of the ball to increase the trajectory speed and regularize control of a tennis ball during its trajectory.
A tennis ball is normally formed by covering the periphery of a rubber sphere with two dumb bell shaped pieces of felt. The felt or "melton" with which a tennis ball is covered is made of a high quality cloth including wool and nylon. The weft (transverse) yarns are made from a wool and nylon mixture which are woven with warp (longitudinal) yarns, which are cotton, in such a way that the weft yarns appear predominately on one side of the cloth. The surfaces of the felt are subject to a teasing or "raising" operation to produce a hairy surface. The hairy surface is then consolidated by a "fulling" process, in which the natural felting properties of wool are exploited by working the cloth in a soap solution to produce the necessary surface texture for the felt.
The felt is coated with a rubber solution on its reverse side and is cut into dumb bell shape covers, two of which are used to cover the spherical surface of a rubber core of the tennis ball. The dumb bell shape covers are usually applied by hand to a rubber core with a reguired degree of stretching. The amount of stretching is carefully controlled so that a complementary fit between the two dumb bell shape pieces of felt is obtained leaving a track of substantially constant width between the two covers. Rubber cement applied to the edges of the dumb-bell shape pieces of felt becomes vulcanized in a further molding operation in which a plurality of balls are heated in spherical molds. A steaming operation again raises the nap on the outer surface of the felt and the finished tennis balls are tested for deformation under a load.
During the securing of the covers to the core, a track is formed between the two covers, spacing the covers away from each other and revealing a narrow surface strip of the rubber core. Due to the thickness and fitting together of the dumb-bell shape felt covers, the depth of the space between the two covers along the track can vary.
During tennis play, dependent upon the court surface, the felt covers of the core fuzz up and wear away. The change in the surface texture of the felt cover members creates various uncontrolled air turbulences around the ball during flight. The effect of the uncontrolled air turbulences on the ball causes the ball to loft or dip more or less than desired by the player. The uncontrolled air turbulences will also affect the speed of the ball along its trajectory. Present day use of tennis balls on hard surface courts, instead of grass courts, increases the amount of wear on the cover members of a tennis ball and decreases the playing life of a ball.